In the prior art, in an industrial or home lock stitch sewing machine using a full rotation hook, either a shuttle bobbin having a lower thread wound thereon or a shuttle bobbin case accommodating the bobbin is mounted in an inner hook, and the inner hook or an outer hook is equipped with a loop-taker point. The hook having the loop-taker point is rotated to guide in the upper thread inserted into a needle so that the upper thread is picked up by the loop-taker point and the upper thread and the lower thread may be interlaced to form lock stitches in a work.
In the case of an outer hook rotation type having the loop-taker point in the outer hook, for example, the inner hook has to be fixed by an abutment between an inner hook groove formed in the inner hook and an inner hook stopper disposed at a frame. Since the inner hook is fixed although the outer hook is rotated at a high speed, the abutment between the inner hook stopper and the inner hook groove is made in a high rotation-friction torque. When the upper thread is guided out from the hook, it is obliged to push the abutment and guide out when it is taken up by a thread take-up lever. As a result, the upper thread is subjected to a far higher inner hook extraction tension than the intrinsically necessary thread pulling tension by the thread take-up lever. This make it impossible to stabilize the interlace point between the upper thread and the lower thread at the time when the upper thread and the lower thread are interlaced to form the lock stitches in the work, so that the seam puckering is caused in the work by the high upper thread tension.
In the case of an outer hook rotation type horizontal hook, therefore, a rotation hook device (as referred to Patent Publication 1, for example) is proposed which is provided with an opener, which rocks at a predetermined timing according to the rotation of an outer hook, and a stopper plate which is attached to the opener and can alternately engage with stopper grooves formed in the inner hook. The stopper plate is extracted from one of the stopper grooves, when the upper thread passes through the stopper groove, and the other stopper groove is brought into engagement to stop the rotation of the inner hook.
There is proposed an opener (as referred to Patent Publication 2, for example), in which the upper thread is guided out from the outer hook by forcibly opening the abutment between the inner hook stopper and the inner hook groove.
There is further proposed an opener driving mechanism (as referred to Patent Publication 3, for example), which can be attached to a general-purpose horizontal hook of a sewing machine and equipped with an opener for rotating an inner hook oppositely to the rotating direction of the outer hook thereby to reduce the rattling noises between the engaging projection of the inner hook and the retaining member of the throat plate.
Those hook device using the opener has drawbacks of complicated mechanisms and serious noises. There is further proposed a full rotation hook (as referred to Patent Publication 4 or Patent Publication 5, for example), in which the inner hook is equipped with a loop-taker point and in which a driver axis is eccentric to the rotation center of the inner hook.
There is also known a fine thread split preventing full rotation hook device (as referred to Patent Publication 6 or Patent Publication 7), in which a needle thread is prevented, when extracted or guided out from the inner hook, from becoming wild, thereby to avoid the piercing of the needle thread by the needle or the shortage of the thread.
Patent Publication 1: JP-A-61-149196;
Patent Publication 2: JP-A-63-115591;
Patent Publication 3: JP-A-2002-143588;
Patent Publication 4: Domestic Re-publication of PCT
Patent application: 2000/73556
Patent Publication 5: JP-A-11-226284
Patent Publication 6: JP-A-53-119153
Patent Publication 7: JP-A-53-125151